Nowadays is an adverb of time similar in meaning to currently or presently. It is written as one word without hyphenation (i.e. nowadays, not now-a-days or now a days).
In your last example sentence, the use of nowadays with any more (or anymore, depending on what side of the debate you fall on) is rather redundant. You could simply say:
Young people nowadays don't respect their teachers.
Young people don't respect their teachers anymore.
Both sentences imply that young people respected their teachers in the past, though that implication is stronger/clearer in the second sentence.